In this SBIR Phase I project, IllExcor Therapeutics proposes the development and evaluation of novel prodrug therapeutics as a yet untapped approach to treat sickle cell disease (SCD), a hereditary malady involving a single point hemoglobin (Hb) mutation that leads to severe adverse physiological complications culminating in painful vaso-occlusive crises, chronic endothelial damage, and progressive end-organ injury and dysfunction, ultimately leading to premature death. The small molecules proposed for therapeutic development are novel, more potent derivatives of the natural product 5-(Hydroxymethyl)furfural (5-HMF), a highly effective antisickling agent that targets the root cause of SCD by allosterically increasing Hb oxygen (O2) affinity. Notably, this compound has demonstrated potent antisickling activity with minimal toxicity in Phase I clinical trials (healthy adults and SCD patients). Additional compounds acting through the same mechanism include vanillin (and derivatives) and GBT440 (Voxelotor, currently in Phase III clinical trials). However, the biological activity of 5-HMF, its derivatives, and all antisickling compounds acting through this mechanism, centers around an aldehyde moiety that forms a Schiff-base with the N-termini of Hb ?-subunits. And while this essential aldehyde is key for bioactivity, it also undergoes rapid in vivo metabolism in most cases, and consequently, is responsible for limiting the plasma half- life and bioavailability of many promising drug candidates. Therefore, the goal of this Phase I proposal is to establish the feasibility and therapeutic potential of the most potent of our novel, metabolically labile thiazolidine ethyl ester prodrug derivatives of 5-HMF. This approach has already demonstrated the potential for more durable in vivo pharmacokinetics in our preliminary studies. The results of the proposed investigation are anticipated to help obtain critical preliminary data to support larger IND-enabling studies in Phase II. To achieve the SBIR Phase I goals, two Specific Aims are proposed. Specific Aim 1: Scale-up chemical synthesis for 3 prodrug derivatives, and a control (i.e., MSDD1, underivatized, native 5-HMF thiazolidine ethyl ester prodrug); and Specific Aim 2: Evaluate the in vivo PK/PD properties of the prodrugs in wild-type mice, and, subsequently, the most promising prodrug candidate in the Townes mouse model of SCD. While the goal of this Phase I proposal is to identify a lead SCD candidate possessing in vivo properties and efficacy to support a Phase II proposal, we anticipate that a more complete understanding of the untapped potential of prodrug derivatization may have a profound impact on the future development of allosteric Hb modifiers for a wide variety of conditions.